Low density foam, midsole, footwear, and methods for making low density foam

ABSTRACT

Foamed thermoplastic elastomeric polyurethane and ethylene-vinyl acetate copolymer articles are made with a combination of a supercritical fluid and a non-supercritical fluid blowing agent.

FIELD OF THE INVENTION

The present invention relates to foamed articles of thermoplasticelastomers and methods for making them.

INTRODUCTION TO THE DISCLOSURE

This section provides information helpful in understanding the inventionbut that is not necessarily prior art.

Thermoplastics are desirable as recyclable materials. However, thermosetmaterials can have properties better suited for some applications.

Brant et al., U.S. Pat. No. 6,759,443 describes polyurethane foam shoesoles made by foaming a polyurethane made from vinyl polymer-graftedpolyoxyalkylene polyether. Polyethylene wax and polytetrafluoroethyleneare added to improve abrasion resistance.

Takemura et al., U.S. Pat. No. 6,878,753 describes shoe soles andmidsoles made of a thermoset polyurethane foam. The foam is made by aprocess comprising mixing a polyol solution, which is previouslyprepared by mixing a polyol, with a catalyst, water and urea, a chainextender, and an additive as occasion demands, with a polyisocyanatecompound with stirring in a molding machine; and injecting the resultingmixture into a mold and foaming the mixture. The density of a moldedarticle of the polyurethane foam is said to be 0.15 to 0.45 g/cm³.

It can be important for cushioning materials to have as propertiesresiliency and high energy return, but thermoplastic elastomersproviding such properties have generally produced foams of higherdensity than is desirable in certain applications.

SUMMARY OF THE DISCLOSURE

This section provides a general summary rather than a comprehensivedisclosure of the full scope of the invention and of all its features.

Disclosed is a method for making a low density foamed article by foaminga thermoplastic polyurethane elastomer or a thermoplastic elastomerethylene-vinyl acetate copolymer using a supercritical fluid. Thethermoplastic elastomer is combined with up to 15% by weight, based onpolymer weight, of a physical or chemical blowing agent other than asupercritical fluid, and this mixture is combined in an extruder with asupercritical fluid. The mixture of the polymer, blowing agent, andsupercritical fluid is either extruded through a die to form a foam thatis cut or molded or both into a desired shape for the foamed article, orthe mixture is injection molded to form the foamed article. In anembodiment for injection molding, the mold contains a porous tool forabsorbing gas generated during foaming of the molded article.

The foamed thermoplastic polyurethane elastomer or ethylene vinylacetate articles may have densities of as low as about 0.15 g/cm³. Themethod may be used to make very low density cushioning components forstraps, protective gear, footwear components (such as a midsole or partof a midsole, an outsole, cushioning for a tongue or a sockliner), andfor other applications.

“A,” “an,” “the,” “at least one,” and “one or more” are usedinterchangeably to indicate that at least one of the item is present; aplurality of such items may be present unless the context clearlyindicates otherwise. All numerical values of parameters (e.g., ofquantities or conditions) in this specification, including the appendedclaims, are to be understood as being modified in all instances by theterm “about” whether or not “about” actually appears before thenumerical value. “About” indicates that the stated numerical valueallows some slight imprecision (with some approach to exactness in thevalue; approximately or reasonably close to the value; nearly). If theimprecision provided by “about” is not otherwise understood in the artwith this ordinary meaning, then “about” as used herein indicates atleast variations that may arise from ordinary methods of measuring andusing such parameters. In addition, disclosure of ranges includesdisclosure of all values and further divided ranges within the entirerange.

Further areas of applicability will become apparent from the descriptionprovided herein. It should be understood that the description andspecific examples are intended for purposes of illustration only and arenot intended to limit the scope of the present disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments are described with reference to the drawings, inwhich:

FIGS. 1A and 1B show an injection molding system used in an embodimentof the disclosed process at different stages during the molding cycle;

FIGS. 2A and 2B are exploded views of the injection mold and theclamping device of the injection molding system of FIGS. 1A and 1B;

FIG. 3 illustrates an extrusion system used in an embodiment of theprocess;

FIG. 4 illustrates a multi-hole blowing agent feed orifice arrangementand extrusion screw that may be used in embodiments of the process;

FIG. 5 illustrates the work surface area and the platen of a mold thatmay be used in the disclosed injection molding process; and

FIG. 6 illustrates an embodiment of a mold that may be used in thedisclosed injection molding process.

DETAILED DESCRIPTION

A detailed description of exemplary, nonlimiting embodiments follows.

The low density foamed article is made by foaming a thermoplasticpolyurethane elastomer or a thermoplastic ethylene-vinyl acetatecopolymer using both a supercritical fluid and up to about 15% byweight, based on polymer weight, of a physical or chemical blowing agentother than a supercritical fluid.

Thermoplastic TPU and EVA Elastomers

The thermoplastic polyurethane elastomer may have a melt index (alsocalled a melt flow index or melt flow rate) of from about 5 to about 100grams/10 min. (at 190° C., 8.7 kg) or from about 180 to about 300grams/10 min. (at 200° C., 21.6 kg) as measured using the procedure ofASTM D1238. The thermoplastic ethylene-vinyl alcohol copolymer may havea melt index of from about 0.5 to about 50 grams/10 min. (at 190° C.,2.16 kg) as measured using the procedure of ASTM D1238. In variousembodiments, the melt index for the polyurethane is preferably fromabout 5 to about 50 grams/10 min. (at 190° C., 8.7 kg), more preferablyfrom about 15 to about 30 grams/10 min. (at 190° C., 8.7 kg) orpreferably from about 180 to about 250 grams/10 min. (at 200° C., 21.6kg), more preferably from about 180 to about 220 grams/10 min. (at 200°C., 21.6 kg) as measured using the procedure of ASTM D1238. In variousembodiments, the melt index for the thermoplastic ethylene-vinyl alcoholcopolymer is preferably from about 2.5 to about 10 grams/10 min. (at190° C., 2.16 kg), more preferably from about 1 to about 10 grams/10min. (at 190° C., 2.16 kg) as measured using the procedure of ASTMD1238.

Thermoplastic polyurethanes can be produced via reaction ofdiisocyanates with difunctional compounds reactive toward isocyanates.In general, the difunctional compounds have two hydroxyl groups (diols)and may have a molar mass of from 62 (the molar mass of ethylene glycol)to about 10,000, although difunctional compounds having otherisocyanate-reactive groups (e.g., secondary amines) may be used,generally in minor amounts, and a limited molar fraction oftri-functional and mono-functional isocyanate-reactive compounds may beused. Preferably, the polyurethane is linear. Including difunctionalcompounds with molar masses of about 400 or greater introduces softsegments into the polyurethane. An increased ratio of soft segments tohard segments in the polyurethane causes the polyurethane to becomeincreasingly more flexible and eventually elastomeric.

The elastomeric thermoplastic polyurethanes may be thermoplasticpolyester-polyurethanes or thermoplastic polyether-polyurethanes.Nonlimiting, suitable examples of these include polyurethanespolymerized using as diol reactants polyesters diols prepared from diolsand dicarboxylic acids or anhydrides, polylactone polyesters diols (forexample polycaprolactone diols), polyester diols prepared from hydroxyacids that are monocarboxylic acids containing one hydroxyl group,polytetrahydrofuran diols, polyether diols prepared from alkylene oxidesor combinations of alkylene oxides, and combinations of these. Theelastomeric thermoplastic polyurethane may be prepared by reaction of atleast one of these polymeric diols (e.g., polyester diol, polyetherdiol, polylactone diol, or polytetrahydrofuran diol), one or morepolyisocyanates, and, optionally, one or more monomeric chain extensioncompounds. Chain extension compounds are compounds having two or morefunctional groups, preferably two functional groups, reactive withisocyanate groups. Preferably the elastomeric thermoplastic polyurethaneis substantially linear (i.e., all or substantially all of the reactantsare di-functional).

Nonlimiting examples of polyester diols used in forming the elastomericthermoplastic polyurethane include those prepared by the condensationpolymerization of dicarboxylic compounds, their anhydrides, and theirpolymerizable esters (e.g. methyl esters) and diol compounds.Preferably, all of the reactants are di-functional, although smallamounts of mono-functional, tri-functional, and higher functionalitymaterials (perhaps up to a few mole percent) can be included. Suitabledicarboxylic acids include, without limitation, glutaric acid, succinicacid, malonic acid, oxalic acid, phthalic acid, hexahydrophthalic acid,adipic acid, maleic acid, anhydrides of these, and mixtures thereof.Suitable polyols include, without limitation, wherein the extender isselected from the group consisting of ethylene glycol, diethyleneglycol, triethylene glycol, tetraethylene glycol, propylene glycol,dipropylene glycol, tripropylene glycol, tetrapropylene glycol,cyclohexanedimethanol, 2-ethyl-1,6-hexanediol, 1,4-butanediol,1,5-pentanediol, 1,3-propanediol, butylene glycol, neopentyl glycol, andcombinations thereof. Small amounts of triols or higher functionalitypolyols, such as trimethylolpropane or pentaerythritol, are sometimesincluded. In a preferred embodiment, the carboxylic acid includes adipicacid and the diol includes 1,4-butanediol. Typical catalysts for theesterification polymerization are protonic acids, Lewis acids, titaniumalkoxides, and dialkyl tin oxides.

Hydroxy carboxylic acid compounds such as 12-hydroxystearic acid mayalso be polymerized to produce a polyester diol. Such a reaction may becarried out with or without an initiating diol such as one of the diolsalready mentioned.

Polylactone diol reactants may also be used in preparing the elastomericthermoplastic polyurethanes. The polylactone diols may be prepared byreacting a diol initiator, e.g., a diol such as ethylene or propyleneglycol or another of the diols already mentioned, with a lactone.Lactones that can be ring opened by an active hydrogen such as, withoutlimitation, ε-caprolactone, γ-caprolactone, β-butyrolactone,β-propriolactone, γ-butyrolactone, α-methyl-γ-butyrolactone,β-methyl-γ-butyrolactone, γ-valerolactone, δ-valerolactone,γ-decanolactone, δ-decanolactone, γ-nonanoic lactone, γ-octanoiclactone, and combinations of these can be polymerized. The lactone ringcan be substituted with alkyl groups of 1-7 carbon atoms. In onepreferred embodiment, the lactone is ε-caprolactone. Useful catalystsinclude those mentioned above for polyester synthesis. Alternatively,the reaction can be initiated by forming a sodium salt of the hydroxylgroup on the molecules that will react with the lactone ring.

In preparing a polyether diol, a diol initiator such as ethylene glycol,propylene glycol, 1,4-butanediol, or another of the diols mentionedabove is reacted with an oxirane-containing compound to produce apolyether diol. The oxirane-containing compound is preferably analkylene oxide or cyclic ether, and more preferably it is a compoundselected from ethylene oxide, propylene oxide, 1-butene oxide,tetrahydrofuran, and combinations of these. Other useful cyclic ethersthat may be polymerized include, without limitation, 1,2-cyclohexeneoxide, 2-butene oxide, 1-hexene oxide, tert-butylethylene oxide, phenylglycidyl ether, 1-decene oxide, isobutylene oxide, cyclopentene oxide,1-pentene oxide, and combinations of these. The polyether polymerizationis typically base-catalyzed. The polymerization may be carried out, forexample, by charging the hydroxyl-functional initiator and a catalyticamount of caustic, such as potassium hydroxide, sodium methoxide, orpotassium tert-butoxide, and adding the alkylene oxide at a sufficientrate to keep the monomer available for reaction. Two or more differentalkylene oxide monomers may be randomly copolymerized by coincidentaladdition and polymerized in blocks by sequential addition.

Tetrahydrofuran may be polymerized by a cationic ring-opening reactionusing such counterions as SbF₆ ⁻, AsF₆ ⁻, PF₆ ⁻, SbCl₆ ⁻, BF₄ ⁻, CF₃SO₃⁻, FSO₃ ⁻, and ClO₄ ⁻. Initiation is by formation of a tertiary oxoniumion. The polytetrahydrofuran segment can be prepared as a “livingpolymer” and terminated by reaction with the hydroxyl group of a diolsuch as any of those mentioned above.

The polymeric diol, such as the polymeric polyester diols and polyetherdiols described above, that are used in making an elastomericthermoplastic polyurethane synthesis preferably have a number averagemolecular weight (determined for example by the ASTM D-4274 method) offrom about 300 to about 8,000, or from about 300 to about 5000, or fromabout 300 to about 3000.

The synthesis of an elastomeric thermoplastic polyurethane may becarried out by reacting one or more of the polymeric diols, one or morecompounds having at least two (preferably two) isocyanate groups, and,optionally, one or more chain extension agents. The elastomericthermoplastic polyurethanes are preferably linear and thus thepolyisocyanate component preferably is di-functional or substantiallydi-functional. Useful diisocyanate compounds used to prepare theelastomeric thermoplastic polyurethanes, include, without limitation,methylene bis-4-cyclohexyl isocyanate, cyclohexylene diisocyanate(CHDI), isophorone diisocyanate (IPDI), m-tetramethyl xylylenediisocyanate (m-TMXDI), p-tetramethyl xylylene diisocyanate (p-TMXDI),ethylene diisocyanate, 1,2-diisocyanatopropane, 1,3-diisocyanatopropane,1,6-diisocyanatohexane (hexamethylene diisocyanate or HDI), 1,4-butylenediisocyanate, lysine diisocyanate, 1,4-methylene bis-(cyclohexylisocyanate), 2,4-tolylene (“toluene”) diisocyanate and 2,6-tolylenediisocyanate (TDI), 2,4′-methylene diphenyl diisocyanate (MDI),4,4′-methylene diphenyl diisocyanate (MDI), o-, m-, and p-xylylenediisocyanate (XDI), 4-chloro-1,3-phenylene diisocyanate, naphthylenediisocyanates including 1,2-naphthylene diisocyanate, 1,3-naphthylenediisocyanate, 1,4-naphthylene diisocyanate, 1,5-naphthylenediisocyanate, and 2,6-naphthylene diisocyanate, 4,4′-dibenzyldiisocyanate, 4,5′-diphenyldiisocyanate, 4,4′-diisocyanatodibenzyl,3,3′-dimethoxy-4,4′-biphenylene diisocyanate,3,3′-dimethyl-4,4′-biphenylene diisocyanate, 1,3-diisocyanatobenzene,1,4-diisocyanatobenzene, and combinations thereof. Particularly usefulis diphenylmethane diisocyanate (MDI).

Useful active hydrogen-containing chain extension agents generallycontain at least two active hydrogen groups, for example, diols,dithiols, diamines, or compounds having a mixture of hydroxyl, thiol,and amine groups, such as alkanolamines, aminoalkyl mercaptans, andhydroxyalkyl mercaptans, among others. The molecular weight of the chainextenders may range from about 60 to about 400 g/mol. Alcohols andamines are preferred in some embodiments; diols are particularlypreferred. Typical examples of useful diols that are used aspolyurethane chain extenders include, without limitation,1,6-hexanediol, cyclohexanedimethanol, 2-ethyl-1,6-hexanediol,1,4-butanediol, ethylene glycol and lower oligomers of ethylene glycolincluding diethylene glycol, triethylene glycol and tetraethyleneglycol; propylene glycol and lower oligomers of propylene glycolincluding dipropylene glycol, tripropylene glycol and tetrapropyleneglycol; 1,3-propanediol, neopentyl glycol, dihydroxyalkylated aromaticcompounds such as the bis(2-hydroxyethyl) ethers of hydroquinone andresorcinol; p-xylene-α,α′-diol; the bis(2-hydroxyethyl) ether ofp-xylene-α,α′-diol; m-xylene-α,α′-diol and the bis(2-hydroxyethyl)ether, 3-hydroxy-2,2-dimethylpropyl 3-hydroxy-2,2-dimethylpropanoate;and mixtures thereof. Suitable diamine extenders include, withoutlimitation, p-phenylenediamine, m-phenylenediamine, benzidine,4,4′-methylenedianiline, 4,4′-methylenibis(2-chloroaniline), ethylenediamine, and combinations of these. Other typical chain extenders areamino alcohols such as ethanolamine, propanolamine, butanolamine, andcombinations of these. Preferred extenders include ethylene glycol,diethylene glycol, triethylene glycol, tetraethylene glycol, propyleneglycol, dipropylene glycol, tripropylene glycol, tetrapropylene glycol,1,3-propylene glycol, 1,4-butanediol, 1,6-hexanediol, and combinationsof these.

In addition to di-functional extenders, a small amount of tri-functionalextenders such as trimethylolpropane, 1,2,6-hexanetriol and glycerol, ormono-functional active hydrogen compounds such as butanol or dimethylamine, may also be present. The amount of tri-functional extenders ormono-functional compounds employed would preferably be a few equivalentpercent or less based on the total weight of the reaction product andactive hydrogen containing groups employed.

The reaction of the polyisocyanate(s), polymeric diol(s), and,optionally, chain extension agent(s) is typically conducted by heatingthe components, generally in the presence of a catalyst. Typicalcatalysts for this reaction include organotin catalysts such as stannousoctoate or dibutyl tin dilaurate. Generally, the ratio of polymericdiol, such as polyester diol, to extender can be varied within arelatively wide range depending largely on the desired hardness of theelastomeric thermoplastic polyurethane. For example, the equivalentproportion of polyester diol to extender may be within the range of 1:0to 1:12 and, more preferably, from 1:1 to 1:8. Preferably, thediisocyanate(s) employed are proportioned such that the overall ratio ofequivalents of isocyanate to equivalents of active hydrogen containingmaterials is within the range of 0.95:1 to 1.10:1, and more preferably,0.98:1 to 1.04:1. The polymeric diol segments typically are from about25% to about 65% by weight of the elastomeric thermoplasticpolyurethane, and preferably from about 25% to about 50% by weight ofthe elastomeric thermoplastic polyurethane.

The polyurethanes may be used in any combination to make the low densityfoamed articles, including using combinations of thermoplastic polyetherpolyurethane elastomers and thermoplastic polyester polyurethaneelastomers.

In certain embodiments it may be preferred to use an elastomer polyesterpolyurethane prepared from a polyester of a dicarboxylic acid havingfrom 4 to about 8 carbon atoms, or its esterifiable derivatives such asanhydride or lower alkyl ester, and a diol having from about 4 to about8 carbon atoms, particularly an unbranched diol. In certain embodiments,it may be preferred to use a polyester polyurethane prepared from apoly(ε-caprolactone) diol polyester or a polyether polyurethane preparedfrom a poly(tetrahydrofuran)polyether [also know as apoly(tetramethylene oxide)polyether or poly(tetramethyleneether)glycol]. These may be used in any combination.

In certain embodiments, preferred polyurethanes in the solid (unfoamed)form may have a Shore A hardness of from about 35 A to about 85 A,preferably from about 50 A to about 75 A, and more preferably from about60 A to about 70 A, as measured by the method of according to ASTMD2240. In certain preferred embodiments, the polyurethane has a Shore Ahardness of from about 50 A to about 80 A or from about 60 A to about 70A and a melt flow index of from about 5 to about 100 grams/10 min. (at190° C., 8.7 kg) or from about 180 to about 300 grams/10 min. (at 200°C., 21.6 kg)

Nonlimiting examples of suitable commercially available polyurethaneelastomers include Elastollan® SP9213 (melt flow index of 200 g/10 min.(at 200° C., 21.6 kg), Elastollan®1170, Elastollan®1180, and a soft 45 Apolyurethane version of these, which are available from BASFPolyurethanes GmbH; Texin® 1209, available from Bayer MaterialScience;and Estane MPD 00361B, available from the Lubrizol Corporation.

Elastomeric ethylene-vinyl acetate copolymers may be prepared byfree-radical emulsion polymerization of ethylene and up to about 50% byweight vinyl acetate. The vinyl acetate monomer is usually at leastabout 10% by weight, preferably at least about 25% by weight of themonomers used. The ethylene-vinyl acetate copolymer has a vinyl acetatecontent of preferably from about 25 weight percent to about 50 weightpercent and more preferably from about 35 weight percent to about 50weight percent. The ethylene-vinyl acetate copolymers may have a meltflow index of from about 0.5 up to about 50 grams/10 min. (at 190° C.,2.16 kg) as measured using the procedure of ASTM D1238. Nonlimitingexamples of suitable commercially available ethylene-vinyl acetatecopolymers include Elvax 265, Elvax 40L-3 from DuPont and Lavaprene 400from Langxess. The ethylene-vinyl acetate copolymers may be used incombination.

Pellets, beads, particles, or other pieces of the polyurethanethermoplastic elastomer or ethylene-vinyl acetate copolymer (EVA) areintroduced into an extruder along with a physical or chemical blowingagent other than a supercritical fluid, with the blowing agent beingused in an amount up to about 15% by weight based on polymer weight. Theblowing agent may preferably be one that contains or produces nitrogenor carbon dioxide. The pellets may have a regular or irregular shape,including generally spherical, cylindrical ellipsoidal, cubic,rectangular, and other generally polyhedral shapes as well as irregularor other shapes, including those having circular, elliptical, square,rectangular, or other polygonal cross-sectional outer perimeter shapesor irregular cross-sectional shapes with or without uniform widths ordiameters along an axis. “Generally” is used here to indicate an overallshape that may have imperfections and irregularities, such as bumps,dents, imperfectly aligned edges, corners, or sides, and so on.

Blowing Agent (Other than a Supercritical Fluid)

The blowing agent may be incorporated into the polymer pellets or may bedry blended with the pellets. In general, the blowing agent may be usedin amounts of from about 1% by weight to about 15% by weight, based onthe total weight of the polymer. In various embodiments, blowing agentor blowing agents other than supercritical fluids may be used in amountsof from about 2% by weight to about 15% by weight, or from about 3% byweight to about 12% by weight, or from about 3% by weight to about 10%by weight, based on the weight of the polymer.

Physical foaming and blowing agents function as gas sources byundergoing a phase change. Suitable physical blowing and foaming agentsmay be selected from the group consisting of aliphatic hydrocarbons andtheir chloro- and fluoro-derivatives. Typical foaming and blowing agentsmay be selected from isomers of pentane, hexane, heptane, fluorocarbons,trichlorofluoromethane, dichlorodifluoromethane,dichlorotetrafluoroethane, monochlorodifluoromethane, and methylenechloride. Chemical foaming and blowing agents produce a gas via achemical reaction. Suitable chemical foaming and blowing agents may beselected from, for example, azo-type compounds for the generation of N₂,ammonium compounds that generate NH₃, and mixtures of carbonates andacids that generate CO₂. Specific suitable examples include sodiumbicarbonate, dinitrosopentamethylene-tetramine, sulfonyl hydroxides,azodicarbonamide, p-toluenesulfonyl semicarbazide, 5-phenyltetrazole,diisopropylhydrazodicarboxylate and sodium borohydrite. The thermaldecomposition of the foaming or blowing agents can be lowered throughaddition of activators or accelerators, as is known in the art.

In another embodiment, microbeads containing physical blowing agent areused as a blowing agent. The microbeads are usually made of a shell ofthermoplastic polymer with, in the core, a liquid, low-boiling gas basedon alkanes. The preparation of these microbeads is described by way ofexample in U.S. Pat. Nos. 3,615,972, 6,509,384, 7,956,096, and8,388,809, the disclosures of which are incorporated herein byreference. The microbeads typically have a diameter of from 5 to 50 μm.Examples of suitable microbeads are obtainable with Expancell® from AkzoNobel.

The chemical or physical blowing agents may be used in any combination.The blowing agent or combination of blowing agents other thansupercritical fluids may be incorporated into the polymer pellets (as inthe common procedure of forming a masterbatch), may be dry blended withthe polymer pellets before being added to the extruder, or may be addedto the extruder separately from the pellets.

Other materials that may be used in thermoplastic elastomericpolyurethane or ethylene-vinyl acetate copolymer compositions that arefoamed include, without limitation, colorants, particularly pigments;fillers, including clays, nanoclays, nanotubes, halloysite clay, whichis available from Applied Minerals under the trademark Dragonite™, andso on; surfactants, release agents, antioxidants, stabilizers,crosslinkers that may be included in amounts that lightly crosslink theelastomer but leave the elastomer foam a thermoplastic, and so on.

Equipment and Process

In describing the equipment and process, “clamping force” refers to theforce that holds together the two mold halves that define the moldcavity. The clamping force can be determined, for example, bymultiplying the hydraulic pressure by the piston surface area in aclamping device (for hydraulic clamping devices), by measuring the forceusing a load cell associated with the mold halves and/or platens, or bymeasuring tie bar elongation or strain and converting to a clampingforce. The term “injection pressure” refers to the pressure at whichpolymeric material is injected into the mold. The injection pressure canbe determined, for example, by measuring the hydraulic load (forhydraulic injection systems) or by measuring the servomotor torque andmultiplying it by the appropriate mechanical advantage factor of thesystem (for electrical injection systems). The term “supercritical fluidadditive” refers to a supercritical fluid under temperature and pressureconditions within an extruder (e.g., 12, FIG. 1). The supercriticalfluid additive may or may not be a supercritical fluid prior tointroduction into the extruder (e.g., when supercritical fluid additiveis in source 22, FIG. 1) and the supercritical fluid additive may beintroduced into the polymeric material in the extruder in any flowablestate, for example, as a gas, a liquid, or a supercritical fluid.

With reference now to the drawings, FIGS. 1A and 1B schematicallyillustrate an injection molding system 10 that may be used in oneembodiment of the disclosed process. An extruder 12 of molding system 10includes a polymer processing screw 14 that is rotatable within a barrel16 to convey the polymeric material in a downstream direction 18 withina polymer processing space 20 defined between the screw and the barrel.A source 22 of supercritical fluid additive that is connected viaconduit 23 to a port 24 formed within the barrel. Extruder 12 includesan outlet 26 connected to an injection mold 28. Optionally, system 10includes a control system 25 that is capable of controlling theinjection molding process and supercritical fluid additive introductioninto the polymeric material.

Generally, injection molding system 10 operates cyclically to producemultiple molded articles. At the beginning of a typical molding cycle,screw 14 is positioned at a downstream end 32 of barrel 16. Thethermoplastic polyurethane elastomer or EVA, typically in pelletizedform, and the blowing agent other than supercritical fluid are fed intopolymer processing space 20 from a hopper 34 through an orifice 36.Barrel 16 is heated by one or more heating units 35 and screw 14 rotatesto knead and mix the melting polyurethane elastomer or EVA and blowingagent and to convey the mixture in downstream direction 18. Thepolyurethane elastomer or EVA and blowing agent mixture should be in afluid state at the point of supercritical fluid additive introductionthrough port 24. The flow rate of supercritical fluid additive into thepolymer mixture may be metered, for example, by a metering device 39positioned between source 22 and port 24. The metering device 39 mayalso be connected to control system 25, which sends signals to controlsupercritical fluid additive flow rate. The molten polymer andsupercritical fluid additive are mixed and conveyed downstream by therotating screw and accumulated in a region 38 within the barreldownstream of the screw. The accumulation of the mixture in region 38creates a pressure that forces the screw axially in an upstreamdirection in the barrel. After a sufficient charge of the mixture hasbeen accumulated, screw 14 ceases to rotate and stops moving in theupstream direction. Preferably, when the screw stops the introduction ofsupercritical fluid additive into the polymeric material is, or hasbeen, stopped, for example, by the operation of an injector valve 40associated with port 24.

Then, the screw is moved axially in a downstream direction by aninjection device 42 to downstream end 32 of the barrel, returning to theoriginal screw position to inject the accumulated charge of the mixturethrough outlet 26 of the extruder and into a cavity 44 (FIG. 2A) definedbetween mold halves 46 a, 46 b. A shut-off nozzle valve 45 associatedwith the outlet of the extruder typically is opened to permit themixture to flow into the cavity. After the charge is injected into thecavity, valve 45 is typically closed. As described further below andshown in FIGS. 2A and 2B, a clamping device 48 holds mold halves 46 a,46 b of the mold 28 (FIG. 2A) together during injection and thesubsequent cooling of the polymeric material. After the polymericmaterial sufficiently solidifies, clamping device 48 separates moldhalves 46 a, 46 b (FIG. 1B) to eject a molded article. The molding cycleis repeated to produce additional molded articles.

The extruder screw extrudes (FIG. 3) or injects (FIGS. 1A, 1B) themixture at a preferred rate of from about 1 to about 5 inches per second(2.54 to about 12.7 cm/s). The mold may be heated, for example up toabout 50° C. The injection pressure may be from about 9000 psi (62 MPa)to about 30,000 psi (207 MPa), preferably from about 18,000 psi (124.1MPa) or from about 22,000 psi (152 MPa) to about 28,000 psi (193 MPa) orto about 30,000 psi (207 MPa), particularly preferably from about 18,000psi (124.1 MPa) to about 28,000 psi (193 MPa).

Molding system 10 may include a number of variations from theillustrative embodiment as known to one of ordinary skill in the art.For example, a mold may define more than one cavity in which articlesmay be molded and may include a hot runner gate to introduce polymericmaterial into the cavities. The hot runner gate may also be providedwith a valve to selectively control introduction of the polyurethane orEVA material. It should also be understood that the injection moldingsystem may be a hydraulic system, an electrical system, or a hybridhydraulic/electric system.

Control system 25 can coordinate the operation of metering device 39with screw 14 so that a desired amount of supercritical fluid additiveis introduced into the polymeric material to form a mixture having thedesired weight percentage of supercritical fluid additive. In someembodiments, a first controller controls the operation of the injectionmolding system and a second controller controls supercritical fluidadditive introduction. In other embodiments, a single controllercontrols operation of the injection molding system and supercriticalfluid additive introduction.

Referring to FIGS. 2A and 2B, injection mold 28 and clamping device 48are shown. In the illustrative embodiment, mold half 46 a is secured toa movable platen 52 a, and mold half 46 b is secured to a fixed platen52 b. Platen 52 a is slideably mounted on a plurality of tie bars 56which extend from a backside 58 of system 10 to fixed platen 52 b.Platen 52 a reciprocates on tie bars 56 to open and close mold 28 inresponse to the action of clamping device 48 which is synchronized withthe molding cycle. Mold 28 is closed when clamping device 48 pushesplaten 52 a in the direction of arrow 60, which forces mold half 46 aagainst mold half 46 b (FIG. 2A). Clamping device 48 holds mold halves46 a, 46 b together with a clamping force during injection and while themolded part cools. To open the mold, clamping device retracts platen 52a in a direction opposite arrow 60 which separates mold halves 46 a, 46b (FIG. 2B).

Other configurations of the injection mold and clamping device may alsobe used in the disclosed process. For example, in some cases, the moldmay not include platens, but rather the movable mold half may be secureddirectly to the clamping device and the other mold half secured to theframe of the system. In other embodiments, a pressure measuring devicemay be associated with mold cavity 44 to monitor pressure within themold (i.e., cavitation pressure). The pressure measuring device may, forexample, access the mold cavity through a wall of one of the moldhalves. The pressure measuring device can send output signalsrepresentative of the cavitation pressure, for example, to controlsystem 25 to control various molding parameters such as injection speedand injection force.

Clamping device 48 may be any suitable type. Clamping device 48 may behydraulically or mechanically/electrically powered. A clamping devicecan be characterized by the maximum force it is capable of providing.Suitable clamping devices may provide a maximum force, for example, ofbetween about 10 tons-force (98 kN) and about 10,000 tons-force (98,000kN), and more typically between about 50 tons-force (490 kN) and about3,000 tons-force (about 29,420 kN). The specific clamping force dependson various factors, such as the article being molded.

Clamping device 48 generally needs to provide a clamping forcesufficient to prevent polymeric material injected into cavity 44 fromflashing between mold halves 46 a, 46 b.

As shown in FIGS. 5 and 6, mold back surface 59 at arrows 1 includes aporous tool 220 in mold cavity 44 adjacent mold wall 210. The poroustool may be located anywhere adjacent an inner mold surface, such asalong the back surface 59 as shown, a side surface 61, or a surface ofthe fixed mold half 46 b, adjacent to more than one area of the innermold surface, or adjacent to all inner surfaces. The porous tool may bean insert that is generally the dimensions of the mold cavity, or may beone or a plurality of smaller inserts—for example, three to fiveinserts-spaced within the mold cavity. The porous tool, or all of theporous tools collectively placed in the mold, may have a pore size offrom about 3 μm to about 20 μm, preferably from about 7 μm to about 12μm, for absorbing gas generated during foaming of the molded article.The porous tool may be a porous metal or metal alloy, such as a porousaluminum or a porous steel, a porous ceramic, such as a porouscordierite, zirconium oxide, silicon carbide, aluminum oxide, or siliconnitride ceramic, or another porous material that does not melt or softenat the temperatures reached during the molding process. Open cell metaland ceramic foams or sintered porous metal materials, which can be madefrom metal powders, can be fabricated as a porous tool in a desiredshape to fit in one or more areas inside the mold cavity and absorb gasduring foaming of the polymer.

The porous tool may also have any desired shape and be placed at adesired location or locations within the mold cavity. For example, theporous tool may have a feature extending into the mold cavity.

In another embodiment shown in FIG. 3, the mixture of polyurethane orEVA, blowing agent, and supercritical fluid additive may be extruded asa foam instead of injection molded. An extrusion system 130 includes ascrew 138 that rotates within a barrel 132 to convey, in a downstreamdirection 33, polymeric material in a processing space 135 between thescrew and the barrel. The polymeric material is extruded through a die137 fluidly connected to processing space 135 and fixed to a downstreamend 136 of barrel 132. Die 137 is configured to form an extrudate 139 ofmicrocellular foam in the desired shape. The thermoplastic polyurethaneelastomer or EVA, for example in the form of pellets containing theblowing agent or as a dry mixture of polyurethane elastomer or EVApellets and the blowing agent, is gravity fed into polymer processingspace 135 through orifice 146 from a standard hopper 144. Extrusionscrew 138 is operably connected, at its upstream end, to a drive motor140 which rotates the screw. Positioned along extrusion barrel 132 aretemperature control units 142, which for example can be electricalheaters or can include passageways for temperature control fluid, thatcan be used to heat the polyurethane or EVA within the extrusion barrelto facilitate melting, to cool the stream to control viscosity, skinformation, or dissolution of the supercritical fluid. The temperaturecontrol units can operate differently at different locations along thebarrel, that is, to heat at one or more locations, and to cool at one ormore different locations. Any number of temperature control units can beprovided. Temperature control units 142 can also optionally be used toheat die 137.

The supercritical fluid additive is introduced into the polymer streamthrough a port 154 in fluid communication with a source 156 of thesupercritical fluid additive. Device 158 can be used to meter thesupercritical fluid additive.

Supercritical Fluid

The supercritical fluid additive may have a variety of compositionsincluding nitrogen, carbon dioxide, and mixtures thereof. According toone preferred embodiment, the supercritical fluid additive is carbondioxide. In another preferred embodiment the supercritical fluidadditive is nitrogen. In certain embodiments, the supercritical fluidadditive is solely carbon dioxide or nitrogen.

Supercritical CO₂ may be combined with the polymer in an amount of fromabout 0.1 weight percent to about 5 weight percent, preferably fromabout 0.5 weight percent to about 5 weight percent, more preferably fromabout 0.5 to about 3 weight percent, and still more preferably fromabout 1 weight percent to about 3 weight percent based on polymerweight. Supercritical N₂ may be combined with the polymer in an amountof from about 0.1 weight percent to about 4 weight percent, preferablyfrom about 0.4 weight percent to about 2.5 weight percent, and morepreferably from about 0.7 weight percent to about 1.5 weight percentbased on polymer weight. When forming microcellular materials, it may bepreferable to form a single-phase solution of polymeric material andsupercritical fluid additive before extruding or injection molding thepolymer mixture. To aid in the formation of a single-phase solution,supercritical fluid introduction may be done through a plurality ofports 24 arranged in the barrel, though it should be understood that asingle port may also be utilized to form a single-phase solution. Whenmultiple ports 24 are utilized, the ports can be arranged radially aboutthe barrel or in a linear fashion along the length of the barrel. Anarrangement of ports along the length of the barrel can facilitateinjection of supercritical fluid additive at a relatively constantlocation relative to the screw when the screw moves axially (in anupstream direction) within the barrel as the mixture of polymericmaterial and supercritical fluid additive is accumulated. Whereradially-arranged ports are used, ports 24 may be placed inequally-spaced positions about the extruder barrel, or in any otherconfiguration as desired. Port 24 (FIGS. 1A, 1B) may include a singleorifice or a plurality of orifices. In the multi-orifice embodiments theport may include at least about 2, and some cases at least about 4, andothers at least about 10, and others at least about 40, and others atleast about 100, and others at least about 300, and others at leastabout 500, and in still others at least about 700 orifices. In anotherembodiment, port 24 includes an orifice containing a porous materialthat permits supercritical fluid additive to flow through and into thebarrel, without the need to machine a plurality of individual orifices.

To further promote the formation of a single-phase solution, port 24 maybe located at a section of the screw that may include full, unbrokenflight paths. In this manner, each flight, passes or “wipes” the portincluding orifices periodically, when the screw is rotating. This wipingincreases rapid mixing of supercritical fluid additive and polymericmaterial in the extruder and the result is a distribution of relativelyfinely divided, isolated regions of supercritical fluid additive in thepolymeric material immediately upon injection into the barrel and priorto any mixing. Downstream of port 24, the screw may include a mixingsection which has highly broken flights to further mix the polymericmaterial and supercritical fluid additive mixture to promote formationof a single-phase solution.

FIG. 4 illustrates an embodiment of the extruder 130 of FIG. 3 in whichthe supercritical fluid additive port is illustrated in greater detailand, in addition, two ports on opposing top and bottom sides of thebarrel are shown. In this embodiment, port 154 is located in theinjection section of the screw at a region upstream from mixing section160 of screw 138 (including highly-broken flights) at a distanceupstream of the mixing section of no more than about 4 full flights,preferably no more than about 2 full flights, or no more than 1 fullflight. Positioned in this way, injected supercritical fluid additive isvery rapidly and evenly mixed into the polymer melt to produce asingle-phase solution of the supercritical fluid in the polymer melt.

Port 154, in the preferred embodiment illustrated, is a multi-hole portincluding a plurality of orifices 164 connecting the blowing agentsource with the extruder barrel. As shown, in preferred embodiments aplurality of ports 154 are provided about the extruder barrel at variouspositions radially and can be in alignment longitudinally with eachother. For example, a plurality of ports 154 can be placed in spacedpositions about the extruder barrel, each including multiple orifices164. In this manner, where each orifice 164 is considered asupercritical fluid additive orifice, there may be at least about 10,preferably at least about 40, more preferably at least about 100, morepreferably at least about 300, more preferably at least about 500, andmore preferably still at least about 700 supercritical fluid additiveorifices in fluid communication with the extruder barrel.

Also in preferred embodiments is an arrangement (as shown in FIG. 4) inwhich the blowing agent orifice or orifices are positioned along theextruder barrel at a location where, when a preferred screw is mountedin the barrel, the orifice or orifices are adjacent full, unbrokenflights 165. In this manner, as the screw rotates, each flight, passes,or “wipes” each orifice periodically. This wiping increases rapid mixingof blowing agent and fluid foamed material precursor by, in oneembodiment, essentially rapidly opening and closing each orifice byperiodically blocking each orifice, when the flight is large enoughrelative to the orifice to completely block the orifice when inalignment therewith. The result is a distribution of relativelyfinely-divided, isolated regions of blowing agent in the fluid polymericmaterial immediately upon injection and prior to any mixing.

Referring again to FIG. 3, a mixing section of screw 138, following thegas injection section, is constructed to mix the blowing agent andpolymer stream to promote formation of a single phase solution ofsupercritical fluid additive and polymer. The mixing section includesunbroken flights which break up the stream to encourage mixing.Downstream the mixing section, a metering section builds pressure in thepolymer-supercritical fluid additive stream prior to die 137. Die 137can have any variety of configurations, as is well known in the art, toproduce microcellular foam in specific forms, for example sheets orprofiles. In addition to shaping extrudate 139, die 137 may also performthe function of nucleating the polymer and supercritical fluid additivesingle-phase solution. The pressure in the single phase solution dropsas the solution flows through the internal passageways of the die. Thispressure drop causes the solubility of the supercritical fluid additivein the polymer to decrease, which is the driving force for the cellnucleation process. The extent of pressure drop depends upon thedimensions of the passageway. Specifically the dimensions that effectpressure drop include the shape of the passageway, the length of thepassageway, and the thickness of the passageway. Typically, the geometryof the die is designed to give a pressure drop suitable for cellnucleation to produce a microcellular foam. Other equipment (notillustrated) downstream of the die is used, as required, for additionalshaping of the extrudate into a final form.

The foamed thermoplastic elastomeric polyurethane article may have adensity of less than about 0.3 g/cm³, preferably less than about 0.25g/cm³, more preferably less than about 0.2 g/cm³. In variousembodiments, the foamed thermoplastic elastomeric polyurethane articlemay have a density of from about 0.15 to about 0.3 g/cm³, or a densityof from about 0.15 to about 0.25 g/cm³, or a density of from about 0.15to about 0.2 g/cm³.

The foamed thermoplastic elastomeric EVA article may have a density ofless than about 0.3 g/cm³, preferably less than about 0.25 g/cm³, morepreferably less than about 0.2 g/cm³. In various embodiments, the foamedthermoplastic elastomeric polyurethane article may have a density offrom about 0.15 to about 0.3 g/cm³, or a density of from about 0.15 toabout 0.25 g/cm³, or a density of from about 0.15 to about 0.2 g/cm³.

The shaped article may be of any dimensions. For example, the moldedarticle may be sized as a cushion or cushioning element that can beincluded in an article of footwear, for example part of a footwearupper, such as a foam element in a collar or tongue, as an insole, as amidsole or a part of a midsole, or as an outsole or a part of anoutsole; foam padding in shinguards, shoulder pads, chest protectors,masks, helmets or other headgear, knee protectors, and other protectiveequipment; an element placed in an article of clothing between textilelayers; in clothing, or may be used for other known padding applicationsfor protection or comfort, especially those for which weight of thepadding or cushioning is a concern. The molded article may beincorporated as cushioning into other articles.

In various embodiments, the molded article is a midsole for an articleof footwear. A midsole provides cushioning in the footwear. A midsoleshould be durable but also preferably adds as little weight as possibleto the footwear while still cushioning to the desired degree. A midsolealso should be able to be bonded to an outsole, an upper, or any othercomponents (e.g., a shank, an airbag, or decorative components) inmaking an article of footwear.

In other embodiments, the molded article is an outsole for an article offootwear.

The invention is further described in the following examples. Theexamples are merely illustrative of various embodiments. All parts areparts by weight unless otherwise noted.

Examples

Injection molded foam parts were prepared by introducing into anextruder pellets of polyurethane elastomers that were dry mixed with ablowing agent (Konz V2894, obtained from BASF), as shown in the table.The blend of blowing agent and polymer are dried and then fed to theinjection machine. The mixture of blowing agent and polymer are thenmelted within the barrel by heat and shear. In the example of theinvention, supercritical fluid carbon dioxide is added into theinjection machine barrel and dispersed within the melted polymer andblowing agent. The mixture of the supercritical fluid carbon dioxide,melted polymer and blowing agent is then injected into the mold. In thecomparative example, the mixture of melted polymer and blowing agent isinjected into the mold. Density of the molded, foamed part is recorded.

Supercritical fluid Blowing Agent, CO₂, wt % wt % (on polymerPolyurethanes, (on polymer Density weight) weight ratio weight) (g/cm³)Example of Konz V2894¹, LJ5913²/SOFT 4 0.28 the Invention 5 wt % 45A³,60/40 Comparative Konz V2894, LJ5913/SOFT 0 0.44 Example 5 wt % 45A,60/40 ¹Obtained from BASF ²LJ5913 has a having a Shore A hardness of 70Aand is a polyether-based TPU, has a melt index in the range of180-300/10 min. @ 200° C./21.6 kg, obtained from BASF. ³Soft 45A ispolyester-based and plasticized TPU having a Shore A hardness of 45A anda melt index in the range of 20-3910/10 min. at 190° C./8.7 kg, obtainedfrom BASF.

The foregoing description of the embodiments has been provided forpurposes of illustration and description. It is not intended to beexhaustive or to limit the invention. Individual elements or features ofa particular embodiment are generally not limited to that particularembodiment, but, where applicable, are interchangeable and can be usedin a selected embodiment, even if not specifically shown or described.The same may also be varied in many ways. Such variations are not to beregarded as a departure from the invention, and all such modificationsare intended to be included within the scope of the invention.

What is claimed is:
 1. A method for making a low density foamed article,comprising: combining in an extruder a molten polymer selected from thegroup consisting of thermoplastic polyurethane elastomers andthermoplastic ethylene-vinyl acetate copolymers with (a) a physical orchemical blowing agent other than a supercritical fluid and (b) asupercritical fluid, wherein the blowing agent (a) is present in anamount up to about 15 wt % based on polymer weight and the supercriticalfluid comprises one of about 0.1 to about 5 weight percent ofsupercritical CO₂ based on polymer weight or about 0.1 to about 4 weightpercent of supercritical N₂ based on polymer weight, to form a mixtureand injection molding the mixture in a mold to form the low densityfoamed article, wherein the mold comprises a porous tool.
 2. A methodaccording to claim 1, wherein the molten polymer comprises athermoplastic polyurethane elastomer having a Shore A hardness of fromabout 35 A to about 85 A.
 3. A method according to claim 2, wherein theShore A hardness is from about 50 A to about 80 A.
 4. A method accordingto claim 1, wherein the molten polymer comprises a thermoplasticethylene-vinyl acetate copolymer having from about 25 weight percent toabout 50 weight percent vinyl acetate content.
 5. A method according toclaim 1, wherein the blowing agent is present in an amount from about 3to about 12 wt % based on polymer weight.
 6. A method according to claim1, wherein the supercritical fluid is one of from about 0.5 to about 3weight percent of supercritical CO₂ based on polymer weight or fromabout 0.4 to about 2.5 weight percent of supercritical N₂ based onpolymer weight.
 7. A method according to claim 1, wherein the mixturefurther comprises a crosslinker.
 8. A method according to claim 1,wherein the mixture is injection molded at a rate of from about 1 toabout 5 inches per second.
 9. A method according to claim 1, wherein theporous tool has a pore size of from about 3 micrometers to about 20micrometers.
 10. A method according to claim 1, wherein the porous toolis a porous metal or metal alloy.
 11. A method according to claim 1,wherein the porous tool is an insert that is generally the dimensions ofthe mold cavity.
 12. A method according to claim 1, wherein the poroustool comprises a plurality of inserts spaced within the mold cavity. 13.A method for making a low density foamed article, comprising: combiningin an extruder a molten polymer selected from the group consisting ofthermoplastic polyurethane elastomers having a melt index of from about5 to about 100 grams/10 min. (at 190° C., 8.7 kg) or from about 180 toabout 300 grams/10 min. (at 200° C., 21.6 kg) and thermoplasticethylene-vinyl acetate copolymers having a melt index of from about 0.5up to about 50 grams/10 min. (at 190° C., 8.7 kg) with (a) a physical orchemical blowing agent other than a supercritical fluid and (b) asupercritical fluid wherein the blowing agent (a) is present in anamount up to 15 wt % based on polymer weight and the supercritical fluidcomprises one of about 0.1 to about 5 weight percent of supercriticalCO₂ or about 0.1 to about 4 weight percent of supercritical N₂, to forma mixture and injection molding the mixture in a mold or extruding themixture.
 14. A method according to claim 13, wherein the molten polymercomprises a thermoplastic polyurethane elastomer having a Shore Ahardness of from about 35 A to about 85 A.
 15. A method according toclaim 13, wherein the Shore A hardness is from about 50 A to about 80 A.16. A method according to claim 13, wherein the molten polymer comprisesa thermoplastic ethylene-vinyl acetate copolymer having from about 25weight percent to about 50 weight percent vinyl acetate content.
 17. Amethod according to claim 13, wherein the blowing agent is present in anamount from about 3 to about 12 wt % based on polymer weight.
 18. Amethod according to claim 13, wherein the supercritical fluid is one offrom about 0.5 to about 3 weight percent of supercritical CO₂ based onpolymer weight or from about 0.4 to about 2.5 weight percent ofsupercritical N₂ based on polymer weight.
 19. A method according toclaim 13, wherein the mixture is injection molded at a rate of fromabout 1 to about 5 inches per second.
 20. A method for making a lowdensity foamed article, comprising: combining in an extruder a moltenpolymer selected from the group consisting of thermoplastic polyurethaneelastomers having a melt index of from about 5 to about 100 grams/10min. (at 190° C., 8.7 kg) or from about 180 to about 300 grams/10 min.(at 200° C., 21.6 kg) and thermoplastic ethylene-vinyl acetatecopolymers having a melt index of from about 0.5 up to about 50 grams/10min. (at 190° C., 8.7 kg) with (a) a physical or chemical blowing agentother than a supercritical fluid and (b) a supercritical fluid whereinthe blowing agent (a) is present in an amount up to 15 wt % based onpolymer weight and the supercritical fluid comprises one of about 0.1 toabout 5 weight percent of supercritical CO₂ or about 0.1 to about 4weight percent of supercritical N₂ to form a mixture and injectionmolding the mixture at an injection pressure of from about 110 MPa toabout 207 MPa into a mold comprising a porous tool.
 21. A methodaccording to claim 20, wherein the molten polymer comprises athermoplastic polyurethane elastomer having a Shore A hardness of fromabout 35 A to about 85 A.
 22. A method according to claim 20, whereinthe blowing agent is present in an amount from about 3 to about 12 wt %based on polymer weight.
 23. A method according to claim 20, wherein thesupercritical fluid is one of from about 1 to about 3 weight percent ofsupercritical CO₂ based on polymer weight or from about 0.4 to about 2.5weight percent of supercritical N₂ based on polymer weight.
 24. A methodaccording to claim 20, wherein the mixture is injection molded at a rateof from about 1 to about 5 inches per second.
 25. A method according toclaim 20, wherein the porous tool has a pore size of from about 3micrometers to about 20 micrometers.
 26. A method according to claim 20,wherein the porous tool is a porous metal or metal alloy.
 27. A methodaccording to claim 20, wherein the porous tool is an insert that isgenerally the dimensions of the mold cavity.
 28. A foamed article madeclaim
 1. 29. A foamed article according to claim 28, wherein the articlehas a density of from about 0.15 to about 3 g/cm³.
 30. A foamed articleaccording to claim 28, wherein the article is a cushion or cushioningelement for an article of footwear, protective equipment, or clothing.